The mineralized tissues are composed of an ordered organic matrix, principally collagen fibers, in which the mineral crystals are deposited. There is a clear relationship between the structure of the matrix and the pattern of deposition of the mineral and the matrix components are always laid down before, or as the first step of normal bone and dentin calcification. The objective of this program is to understand the factors which lead to this orderly deposition of mineral within the matrix. It appears that non-collagenous components of the matrix, phosphophoryns in dentin, interact with collagen at the mineralization front and, because of their calcium binding properties, fix the first layer of calcium ions on which further crystal growth can occur. Present investigations deal specifically with: (1) the properties of the phosphophoryns; (2) the biosynthesis and in particular the secretory pathway of the phosphophoryns; (3) the nature of the interaction between collagen and phosphophoryn and (4) the nature of phosphophoryns (or analogous proteins) in bone.